I. Field of the Invention
This invention relates generally to implantable body tissue stimulating apparatus, and more particularly to an improved configuration for a capacitor used therein for periodically dumping a charge from a battery to tissue to be stimulated under control of an electronic switching circuit.
II. Discussion of the Prior Art
Implantable tissue stimulators of various types are known in the art for delivering electrical stimulating pulses to selected tissue structures. For example, cardiac pacemakers and implantable cardiac defibrillators have been developed for maintaining a desired pacing rate or for treating serious arrhythmias. While other tissue stimulating devices are known for treating a variety of conditions, the present invention will be described in relation to cardiac defibrillators, but it is not intended that the invention be limited to that particular application.
In its simplest form, an implantable cardiac defibrillator typically includes a metallic housing that is hermetically sealed and, therefore, impervious to body fluids. Contained within the housing is a battery power supply, electronic circuitry for detecting pathologic and/or non-pathologic arrhythmias and an electronic switching circuit for dumping a charge built up on a capacitor bank to cardiac tissue, via a set of defibrillating leads.
It is widely understood that life-saving implantable defibrillators need to be small. Making small devices that deliver large amounts of energy has typically been an inversely proportional relationship. The capacitors currently used in implantable defibrillators at least over the past ten years are typically cylindrical in shape. This shape and construction tends not to be volume efficient. There are spaces at the ends of each such cylindrical capacitor and along each side thereof that necessarily goes unused, limiting the ability to reduce the size of the housing. These spaces exist because the capacitor geometry transitions sharpely along corners and edges, while biocompatability requirements necessitate more gradual transitions in the housing to prevent it from causing tissue erosion at the implant site. The energy density and shape of the cylindrical capacitors significantly restricts the ability to design physiologically compatible housings that are extremely energy efficient.
Recent efforts have typically been made to fabricate flat, or cubic, capacitors (capacitors with a rectangular cross-section). These capacitors have been aluminum electrolytic in construction or ceramic, thin film or other known technologies. Like the cylindrical shape, the flat rectangular shape has the same inefficient abrupt corners and ends.
The Lynn U.S. Pat. No. 5,370,663 discloses a capacitor construction for an implantable defibrillator which is intended to conserve space and allow a thinner profile than can be achieved using cylindrical or cubic capacitors. Here, a capacitor is formed by spirally winding elongated aluminum foil strips separated from one another by an insulating paper layer in an oval shape conforming somewhat to the oval shape of the metal housing of the defibrillator. The center portion of the helix is open and permits specially shaped, relatively flat batteries and circuitry to be inserted therein.
Another approach that purports to reduce the overall thickness dimension of an implantable defibrillator device is described in the Kroll U.S. Pat. No. 5,527,346. Here, relatively thin sheets of a suitable polymer that are first provided with a deposited metallic coating on the opposed surfaces thereof are stacked with separate intermediate thin polymer sheets as insulators to fabricate a capacitor of a desired capacitance value. Because of the fabrication techniques employed, the polymer thin film dielectric layers must have a thickness in the range of between 1 and 10 micrometers so as to be able to withstand the handling during fabrication of the capacitors. Films that are handled multiple times due to the processing steps involved, such as casting, stretching, slitting, metallizing, clearing and winding or stacking requires such layer thickness to withstand the stresses imposed during these operations. Films of lesser thickness are likely to have pinholes or other defects imparted to them due to processing stresses, leading to reduced voltage breakdown strength. Moreover, when attempting to work with films thinner than one micron one is presented with even greater difficulties in terms of processing and handling such thinner films. The Kroll patent attempts to minimize capacitor volume by utilizing a relatively high voltage and lower capacitance than is otherwise typically employed in cardiac defibrillating systems.
The Cichanowski U.S Pat. No. 4,586,111 describes a capacitor construction with high volumetric efficiency in which a metal substrate has a base coat of a pre-polymer layer applied to it and this layer is then polymerized using electron beam bombardment. The polymerized layer is about 3-6 microns in thickness. Next, a vacuum deposited aluminum electrode of a thickness in the range of from 300 to 500 Angstroms is applied. The patent also suggests that monolithic multi-layer capacitors may be produced by depositing alternating electrode and dielectric layers so as to provide alternate electrode layers with portions projecting from the stack and contacting each other in electrically connected relation. The dielectric coating between each layer is preferably a polymer of polyacrylate which is formed by the vapor deposition of the pre-polymer and subsequent polymerization thereof.
Various techniques are known in the art for vapor depositing polyacrylates and, in this regard, reference is made to the Shaw et al. U.S. Pat. Nos. 5,440,446 and 5,032,461.
It is a principal object of the present invention to provide an improved capacitor arrangement in an implantable cardiac stimulating device that occupies significantly less volume while still permitting the device to deliver ample amounts of energy. An important benefit derived from the improved capacitor arrangement is the ability to design a housing having improved physiologic compatibility.